Slicing through milky green waters, a Colombian navy patrol wove through the maze of mangroves in the remote Sanquianga national park on the Pacific coast, following a tip.

After eight days, the search paid off. Hidden deep within the boa-infested swampland, the patrol came upon a 60ft hull propped up on a scaffold under a tin-roofed hangar. This was no ordinary shipyard, and it was no ordinary vessel.

Shipbuilders had been putting the finishing fibreglass touches to the hull of what is known here as a narco-sub. Had they finished, the vessel would have been loaded with as much as four tonnes of cocaine and put to sea, headed north, to the US market.

It has been a triumphant month for Colombian navy patrols trying to make a dent in what has become a booming cottage industry: narco-sub shipbuilding. Four narco-subs were discovered in the Sanquianga park operation alone and another two were found on the Caribbean coast.

But it could also mean that the production and use of the subs is on the rise. Eleven have been seized or destroyed so far this year, one quarter of all the narco-subs detected since the first one was caught in 1993.

Traffickers shipping cocaine from South America are resorting to ever more ingenious methods: last week, drugs were found in a haul of frozen shark carcasses.

But it is narco-subs that carry the greatest tonnage – possibly as much as a third of all Colombia's cocaine exports, estimated at 600 tonnes a year.

They are designed to ride low, with only about a foot of the vessel above water so the captain can see where he's going through Plexiglas windows. The hulls are shaped to cause minimum wake and the exhaust pipes snake out from the engine room and down into the water to minimize the thermal signature.

"Once the semi-subs are out at sea it's 98% impossible to detect them," says Major Raúl Donado of Colombia's marines, based in the southern Pacific coast city of Tumaco.

On the rare occasions they are detected at sea, crews typically open an emergency valve built into the subs to scuttle the vessels and their cargo.

With the evidence of cocaine at the bottom of the sea, officials are obliged by international law to treat the crew as castaways, since the vessels themselves are not illegal in Colombia.

"If we don't find drugs or evidence of drugs in the seizure there is no crime," says Bejarano. "The judges have to let the criminals walk." Last year, 21 people captured in connection with drug subs were released.

That may change, however, with a bill passed in the Colombian congress last week that makes it illegal to build, transport or possess unregistered semi-submersible vessels.

Another US law, passed last year, outlaws unregistered submersible or semi-submersibles in international waters. Colombian and US authorities hope the threat of prison will help to deter crews from agreeing to embark on the gruelling journey to transport the drugs.

But Miguel Angel Montoya, a former drug trafficker who says he met more than a dozen crews before they set off on their journey, says the new law will probably have little effect.

"I don't think anything will change, because the organisations take advantage of the poverty in Colombia to lure crew members to make the trip for $10,000 or $20,000," says Montoya, a Mexican who was involved with the Colombian and Mexican drug cartels until 2004. Captains are better paid at about $50,000-$60,000.

Montoya says the four- or five-man crews he met in the jungle-covered shipyards went through a ritual the night before they set off. "They would pray to the Divine Child and to the Virgin, they would be given a hearty meal. It was like they were on death row," he says, adding that many crews were lost at sea.

The crews often refer to the subs as "the can" or "the tube". Coastguard lieutenant Oscar Calderón calls them coffins. "The crew members must be desperate to climb into one of those," he says.

The cabins of the subs measure about six square metres, where usually four men make the two-week journey: a captain, a machinist, a navigator and a cargo representative who makes sure the cocaine reaches the buyer at the other end.

For the duration of the trip they eat canned sausages and tuna and drink Gatorade and Red Bull energy drinks.

To relieve themselves they have to climb out of the cabin and tie themselves to the sub so they do not fall into the sea. The crew alternate sleeping in two bunk spaces on either side of the cabin. Despite exhaust systems, the cabins often fill with diesel fumes.

Since 2007 the sub makers appear to have settled on one standard design and production has begun en masse, controlled by four competing organisations that sell the vessels to the traffickers.

A single sub that slips through the defences can carry as much as 10 tonnes of cocaine. At a price of about $25,000 a kilo wholesale in the US, that means the subs can end up carrying as much as $250m-worth of merchandise at a time.

"We make this huge effort to seize four, but with one that gets through, the drug traffickers make up their losses," says Calderón. "That's what makes our job so frustrating."